There are now a number of systems which operate at high data rates such as multimedia applications like Wireless Local Area Network (WLAN); Wireless Personal Area Network (WPAN), e.g. Bluetooth; etc. Various arrangements have been proposed in order to facilitate such high data rates in a reliable and practical way. However, there are a number of problems associated with high data rate wireless channels, particularly due to multipath. This is especially problematic indoors where the environment is particularly dispersive due to the large number of objects and surfaces as well as the dynamic nature of structures, particularly people, moving about. Consequently, it can be extremely difficult to extract and retrieve the original information from a transmitted signal reliably and without extremely complex processing. This results in complex receivers which must be capable of estimating and compensating for the multiple versions of the original signal arriving at the receiver due to the different path length of each version.
In order to counter this, a multi-carrier approach has been proposed in which the original data stream is separated into a series of parallel data streams, each of which is modulated and transmitted with a different frequency, generally within the same band. This allows the relative size of the transmitted symbols relative to the multipath delay to be much larger and so inter symbol interference is reduced. One particularly advantageous system, which utilises multiple carriers is Orthogonal Frequency Division Multiplexing (OFDM).
OFDM is very effective at overcoming the problems of fading and multipath. This is achieved by dividing a frequency selective fading channel (i.e. a channel where the fading characteristics at one frequency are likely to be different to those at neighbouring frequencies such that the profile of the received signal against frequency is not flat) into a number of flat fading sub-bands. In this way, the profile within the sub-bands is approximately flat. These sub-bands relate to the OFDM sub-carrier frequencies.
FIG. 1 shows an example of the layout of a transmitter 10 and receiver 20 for an OFDM system. In the multi-antenna receiving unit 21, each of the antennas receives a signal, which is fed to an analogue to digital converter 22 and then into a serial to parallel converter 23 to separate the individual sub-channels. The sub channels are then processed through a Fast Fourier Transform (FFT) 24. Finally, the signals are converted from a plurality of parallel signals into a serial signal for each sub-channel and the coded data extracted.
In such an OFDM receiver system, it is possible to apply adaptive beamforming weights at various points in a receiver as shown in FIG. 1. However, the effectiveness of these weightings will largely depend on the stability and coherency of the propagation channel. If the channel undergoes flat fading (i.e. the signal strength of each of the sub-carriers is affected to the same extent), then it can be regarded as being a narrowband channel. In this case, a single set of weights can be applied at radio frequency (RF) or intermediate frequency (IF), to the received signal just after the antenna array 21, i.e. at position (1) in FIG. 1. Alternatively, the weights may be applied after the analogue to digital unit 22 at position (2) in FIG. 1. Both of these positions should be sufficient for optimum spatial processing.
However, in wideband systems, operating at high data rates such as WLAN, WPAN, etc., where bandwidths of 10 MHz or higher may be required and/or systems operating in highly dispersive environments, signals will occupy a spectrum in excess of the coherence bandwidth. Consequently, there will be significant variation in the quality/signal strength of the channels across the bandwidth. Consequently, it is unlikely that a single set of weights (i.e. as in narrowband beamforming) would be satisfactory for beamforming.
One way to overcome this problem is to process the received data and apply weightings in the receiver for each sub-carrier, after the FFT 24, i.e. at position (3). FIG. 2 shows an example of a receiver. In this system, the signal is received by antennas 101,102,103. Pre-processing units 104,105,106, carry out downconversion, A to D conversion, serial to parallel conversion and FFT processing. The outputs are then fed into an array of adaptive signal processing devices 107,108,109 which include a plurality of multipliers 110 which multiply each of the received signals by a weighting value w determined by a weight determining unit 113. Each of the weighted signals from the multipliers is then summed 111 to provide an output signal. The output signals from each of the weighting units is then fed to a combining unit 112 which extracts a data signal in which the delayed signals and interference signals have been removed from the received signal.
However, in the example shown, the receiver has L antennas and the number of sub-channels that each antenna receives is N. Therefore, the total number of weighting units required is L×N. This can lead to a very large number of multipliers 110 being required. For example, in the HIPERLAN system, there are 48 data sub-carriers and 4 pilot sub-carriers (N=52); there is also a DC channel (CH0) which does not carry data. This means that the receiver is complicated and this results in the receiver being expensive and potentially subject to reliability problems. In addition, the weighting is normally implemented in software and so processor demand is extremely high, again resulting in high expense or poor performance. If the processing to determine the weighting to be applied is unduly complex, then it may take a significant amount of time to complete. During this time, the channel parameters may have changed significantly and so the calculated weightings could be inappropriate. Under these circumstances, the weighting produced would always out of date and hence poor performance will result where the characteristics of the channel change rapidly with time.
One way to reduce the processor demand, is to divide the operating bandwidth into a number of sub-bands and then select one sub-carrier from within each sub-band on which to base all calculations. This method relies upon each sub-band behaving generally as a narrowband, i.e. that the sub-band effectively undergoes flat fading. In other words the chosen sub-carrier is accurately representative of the fade within the sub-band as a whole. However, without prior knowledge of the operational environment, it is difficult to know to what extent the operating band should be divided up. Where large sub-bands are chosen there is a danger that the chosen sub-carrier would not be sufficiently representative of the sub-band and performance would be degraded. In contrast, if the number of sub-bands is chosen to be large (i.e. few carriers per sub-band), whilst the representative sub-carrier is likely to be accurately representative of the sub-band, the amount of processing required is disadvantageously high.
EP-A2-0,852,407, which relates to current standards for 5 GHz WLANs, suggests reducing the total number of adaptive signal processing units and hence the number of weighting units to improve the receivers by reducing the complexity. The document describes dividing the operating band into four equal sub-bands each having a ‘pilot’ sub-carrier.
Providing pilot sub-carriers at equal intervals amongst the sub-carriers allows the receiver to calculate the average and differential carrier phase errors. This is achieved by knowing the form of the pilots. Using this information, the receiver can fine-tune the carrier and timing tracking circuits such that errors are minimised.
An example of this arrangement is shown in FIG. 3 where the operating band is divided into fifty-three channels or sub-carriers (i.e. as in HIPERLAN), these are then divided up into four separate groups each defining a sub-band. Each sub-band includes a sub-carrier, which acts as a pilot for the group (in the example, channels −21, −7, 7 and 21). The pilot channels do not carry signal data but contain a predetermined sequence for use in equalising the received signal by comparing the received signal to an expected signal. Weighting for the received signals is determined using the pilot sub-carriers and is then applied to each sub-carrier in the respective sub-band. As indicated above this system relies upon flat fading over each sub-band, which in the case of the above referenced document are of the order of 5 MHz in size.
To aid the coherent demodulation process for OFDM systems, pilot sub-carriers are interspersed between the data sub-carriers at equal intervals. Knowing the form of these pilots, it is possible to calculate the average and differential carrier phase errors, which can in turn be used to fine-tune the carrier and timing tracking circuits in the receiver, such that any errors are minimised.
If the bandwidth of the system is increased such that the sub-bands have considerably greater bandwidth, for example in the region of 10 MHz, then the likelihood that the sub-bands will have flat fading is considerably reduced, particularly where the environment is such as to give strong multipath interference, e.g. indoors. Consequently, the pilot is less likely to provide an accurate indicator for the sub-carriers in the sub-band.
Dynamic allocation of the modulation scheme of sub-carriers has been proposed, for example the Atheros system, to extend the basis of 5 GHz systems to allow a wide range of devices to coexist under a unified protocol. In this way, individual sub-carriers are allocated a modulation scheme according to the existing conditions. In the Atheros system, the highest order modulation scheme possible is allocated to each sub-carrier to maximise throughput. If some of the sub-carriers are experiencing more severe fading, then a lower order modulation scheme can be applied. This reduces the potential throughput but provides a more reliable transfer and so the valid data throughput is maximised.
A further problem associated with this arrangement is that if a pilot channel falls in a frequency slot that is experiencing severe fading, the pilot may be unrecoverable and cannot be used in the calculation of the average and differential carrier phase errors. Consequently, a large portion of the band may become unrecoverable due to the reduced estimation and interpolation between the remaining pilots. FIG. 4 shows an example of a dispersive channel based upon the ETSI 5 GHz HIPERLAN/2 channel models, indicating the approximate spacing and locations of pilots. Of the four pilots, one of the pilots is in a deep fade. Consequently, the receiver may not be able to recover the pilot and therefore the group of channels that rely upon the information from this pilot may also be unrecoverable. Referring back to FIG. 3, it will be apparent that the sub-carriers in group 3 would be unrecoverable due to the loss of the pilot in channel 7.
Application number GB0108026.6 describes a system for rearranging the sub-bands according to the channel characteristics. In this way, the sub-carriers can be dynamically allocated to sub-bands so that all the sub-carriers within a sub-band have similar coherency within a limited period of time.